The invention relates to a structural component made of a metallic or ceramic material having a coating of hard material, and to processes of preparing these.
Structural components that are subjected to high loads, in particular caused by wear, can be protected by a coating of hard material. In particular the tribochemical and thermochemical reactions between the materials of the work piece and the tool must be largely avoided so that the good mechanical and thermal properties of the material may be exploited to the full. It is therefore already known to improve the wear resistance of, for example, metal-cutting tools by means of a coating of hard material. The coatings are applied not only to metal-removing tools made of metal, in particular high-speed steel (HSS) or hard metal (HM), but also to tools made of ceramic materials, such as, for example, silicon nitride blanking dies. Wear can also be reduced by a coating of hard material in the case of metal-forming tools.
Layer thicknesses within the micrometer range generally suffice for the desired property improvements, in particular in the case of metal-cutting tools. Such layers can be deposited, for example, from the gas phase by the PVD, CVD or plasma CVD processes. Depending on the material, layer thicknesses of a layer of less than 1 μm to over 20 μm are possible by these means.
The processes indicated enable not only single layers of one material to be deposited. Many variations are possible as regards both the number of materials which may be combined together in the layer sequence, and the number of layers. Above all when metal-cutting tools are utilised, layer materials based on titanium, such as, for example, titanium nitride, titanium carbide, titanium oxide, titanium carbon nitride are preferably utilised, as well as layers of one or more modifications of aluminium oxide.
Differences in thermal expansion between the substrate material and the layer material, as well as different layer materials in the case of a coating comprising a plurality of layers, may give rise to high stresses. In particular with increasing coating thickness the stress increases, as consequently does the risk of the coating chipping off.
In order to improve the adhesion strength of a coating it is known, for example from DE 36 08 734 C1, in the case of coated ceramic indexable inserts to deposit an intermediate layer of silicon dioxide on a base body made of sintered ceramic materials which is provided with a ceramic wear layer. This prior art, it must be noted, relates only to coating with one layer.
The object of the present invention is to increase the adhesion strength of the layers of hard material on the substrate material of a structural component made of a metallic or ceramic material as well as that between the layers.
The object is achieved according to the present invention, which relates to a structural component of a metallic or ceramic material coated with a hard material. The coating is a multilayer coating.